Lemna minor

The common duckweed (Lemna minor) is the most common and best-known duckweed species in Europe. It occurs mainly in the northern hemisphere (North America, Europe, Asia), but also in the southern hemisphere, including Australia and New Zealand. L. minor often grows in masses on the surface of nutrient-rich, muddy, standing and slowly flowing waters such as pools, ponds, small lakes and ditches. The “duckweed” carpet can become several centimetres thick. The common duckweed often occurs together with other duckweeds such as Spirodela polyrhiza or the submersed Lemna trisulca, which floats beneath the duckweed cover. Similar species that occur wild in Central Europe are the gibbous duckweed (Lemna gibba), the red duckweed (L. turionifera) and the least duckweed (L. minuta).

Duckweeds in aquariums are mostly called Lemna minor. However, they may also be other Lemna species, some of them introduced from the tropics, such as Lemna aequinoctialis. L. minuta and L. valdiviana. The dotted duckweed (Landoltia punctata) also frequently turns up in aquariums and is mistaken for Lemna minor. Identifying duckweed species is often very difficult. All Lemna species have one root per frond. In this the genus Lemna differs from Landoltia and Spirodela (a tuft of several roots per frond) and from Wolffia and Wolffiella (rootless).

Some characteristics of L. minor:

• fronds flattened (not thickened as in Lemna gibba), with a roundish to elliptic-obovate outline, 2 to 4 mm long,
• upper surface smooth, without a row of bumps or a ridge in the middle (as in some other Lemna species),
• three veins are visible in transmitted light (Lemna minuta, e.g., has only one vein).

For further information on duckweeds as aquarium plants see Lemna spp.